How to verify ID Token without requiring credentials?

As per the docs: https://firebase.google.com/docs/auth/admin/verify-id-tokens#c

```csharp
FirebaseToken decodedToken = await FirebaseAuth.DefaultInstance
.VerifyIdTokenAsync(idToken);
string uid = decodedToken.Uid;
```

However this requires initializing the DefaultInstance with service credentials. Verifying the token should be possible without private credentials (if we don't check revoked).

How can I do this?